Prior Art: Wireless Chip Sets
Note that the term xe2x80x9cwirelessxe2x80x9d is generally used hereinbelow in place of the term xe2x80x9cradioxe2x80x9d as being more descriptive of the related technology.
Use of wireless frequencies and communications methods which do not require individual device licensing is preferred. The following is taken from Reference 1 hereinbelow; Intersil Application Note AN9804.1 by Jim Zyren and Al Petrick, dated June 1998:
Spread Spectrum Radios
The term xe2x80x9cspread spectrumxe2x80x9d simply means that the energy radiated by the transmitter is spread out over a wider amount of the RF spectrum than would otherwise be used. By spreading out the energy, it is far less likely that two users sharing the same spectrum will interfere with each other. This is an important consideration in an unlicensed band, which is why the regulatory authorities imposed spread spectrum requirements on radios which transmit over xe2x88x921 dBm (about 0.75 mW).
The frequency allocation by countries is found in Reference 2 hereinbelow; Intersil Application Note AN9829 by Jim Zyren and Al Petrick:
Again from Reference 1:
In the U.S., license-free bands are collectively designated as Industry, Science, and Medicine (ISM) bands. Operation in these bands with approved devices does not require an FCC license. By waving licensing requirements, these bands have been made generally accessible to virtually everyone. This is mainly why ISM bands are so important for commercial and consumer applications.
As mentioned above, radios employing spread spectrum methods are allowed to radiate up to 1.0W (30 dBm) of RF energy, as compared to less than 1 mW for non-spread radios. There are two common types of spread spectrum systems. The easiest to understand is Frequency Hopped Spread Spectrum (FHSS). In this method, the carrier frequency hops from channel to channel in some pre-arranged sequence. The receiver is programmed to hop in sequence with the transmitter. If one channel is jammed, the data is simply retransmitted when the transmitter hops to a clear channel. The major drawback to FHSS is limited data rate. In the 2.4 GHz band, FCC regulations require that the maximum occupied bandwidth for any single channel is 1 MHz. This effectively limits the data rate through this type of system to about 1 Mbps.
By contrast, Direct-Sequence-Spread-Spectrum (DSSS) systems in the ISM bands provide much higher data rates. DSSS systems do not jump from frequency to frequency. Instead, the transmitter actually spreads the energy out over a wide portion of the RF spectrum. This is accomplished by combining the data stream with a much higher rate Pseudo Random Numerical (PRN) sequence via an exclusive xe2x80x9corxe2x80x9d (XOR) function. The result is a digital stream at the same rate as the PRN. When the RF carrier is modulated by the higher speed digital stream, the result is a spreading of the RF energy . . . . At the receiver, the pseudo random code is used to xe2x80x9cde-spreadxe2x80x9d the received data . . . . It is during this process that the matched filter rejects unwanted interference because it is uncorrelated with the PRN. By careful selection of the PRN sequence, the matched filter provides an additional benefit. It can reject multipath signals which are delayed relative to the main signal . . . by 44 ns (or more).
At this time of writing Intersil has introduced their PRISM II-11 Mbps (Mega bits per second) chip set using DSSS technology supporting IEEE Standard 802.11 2.4 GHz Wireless Local Area Networks (WLANs). Moreover this chip set also supports peer-to-peer ad hoc networks.
The following partial specifications are published by the Intersil website:
Variable data rates: 11, 5.5, 2, 1 Mbps
Frequency band: 2.4 GHz ISM Band
Dual modes: 1xe2x80x94WLAN, and
2xe2x80x94Independent Basic Service Sets (IBSSs) consisting of two or more stations (IEDs) which have recognized each other and have established communications. Within an IBSS, stations communicate directly with each other on a peer-to-peer level. Selectively, groups of IBSSs then are combined through a distribution system to an Access Port (AP) to communicate with wired LANs.
Again the following is selectively quoted from Intersil tutorial AN9829 dated February 1999 by Jim Zyren and Al Petrick, reference 2 below, on IEEE standard 802.11 which the PRISM II chip set supports:
DSSS is the same technology used in GPS satellite navigation systems and in CDMA (Code Division Multiple Access) cellular telephones . . . the data stream is combined via an XOR function with a high-speed PRN sequence using an 11 chip Barker Code. The term xe2x80x9cchipxe2x80x9d (note: not to be confused with a chip set of components) is used instead of xe2x80x9cbitxe2x80x9d to denote the fact that the Barker Code does not carry any binary information by and of itself. The result is an 11 Mbps digital stream which is then modulated onto a carrier frequency using Differential Binary Phase Shift Keying (DBPSK).
WLAN radios are half duplex and cannot receive while transmitting. Therefore a collision cannot be detected by a radio while transmission is in progress. The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). Stations (STAs/IEDs) sense the medium to determine if it is idle. If not each STA waits until transmission stops, and then enters into a random back-off procedure. This reduces the probability of data clashes. Packet reception in DCF requires acknowledgement (ACK) . . . The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS).
Fast acknowledgement is one of the salient features of the 801.11 standard, because it requires ACKs to be handled at the Multiple Access Communications (MAC) sub-layer. Transmissions other than ACKs must wait at least a DCF inter-frame space (DIFS) before transmitting data . . . STAs wishing to transmit must wait an integer number of Slot Times depending on an internal timer setting (0 to 7 on first attempt). Upon expiration of a DIFS, the timer begins to decrement and on reaching zero the STA begins transmission. If a collision is detected the window is increased to 15 Slot Times and doubled after each unsuccessful attempt up to a maximum value of 256 Slot Times.
This method relies on the ability of each STA to sense all others . . . . If not the probability of collision is greatly increased. This is known as the Hidden Node. In order to combat this problem, a second carrier sense mechanism, Virtual Carrier Sense is described in the standard. Virtual Carrier Sense is implemented by reserving the medium for a specified period of time for an impending transmission.
The standard, supported by PRISM II, provides for security by two methods: authentication and encryption. It further supports synchronization of STA clocks by periodic transmissions of beacons containing time stamp information. It further supports power management by defining awake and doze modes of operation.
In addition . . . a waveform supporting 20 to 30 Mbps is under development. A task force is drafting a standard based on Orthogonal Frequency Division Multiplexing for implementation of these higher data rates.
A SXT810 chip by Level One, as given by References 4 and 5 hereinbelow, appears to have the advantage of being a single chip however, being designed for use in cordless telephones, it has the disadvantage of a limitation to approximately a 40 KHz data rate and non-compliance with IEEE standard 810.11.
The chip sets described hereinabove are typical of those at the time of writing. Additional chip sets and single chip devices are expected to become available. It is to be understood that the invention contained herein is not limited to the chip sets described herein.
Prior Art: The Electric Power Industry
Electric power distribution substations supply approximately 80% of the power in the U. S. A. directly over distribution lines and cables feeding many homes and businesses. The remaining 20% consist of large entities, such as industries and shopping malls, that are supplied by individual connections to electric power transmission or subtransmission lines.
Intelligent Electronic Devices (IEDs) generally consist of control IEDs for LTC transformers and regulators for the control of voltage as well as protective relay IEDs for operation of devices such as circuit breakers for the interruption of fault currents; the operation commonly being known as xe2x80x9cclearing a faultxe2x80x9d. The term xe2x80x9cintelligent electronic devicesxe2x80x9d is commonly used throughout the electric power industry as the name for control and protection devices and is so used herein. Alternatively the abbreviated term xe2x80x9cIEDxe2x80x9d is used to refer to such devices.
Since the writing of the patent application which resulted in issuance of U.S. Pat. No. 5,943,202 (see Reference 11 hereinbelow), some electric power substation control has been established wherein information has been obtained from unattended stations and fed to central locations by Supervisory Control And Data Acquisition (SCADA) systems. At these central locations, combinations of operators and central computers function together for operation of substations.
In practice the term xe2x80x9cprotocolxe2x80x9d has been extended to include not only the methods of data communications but also the list of data points communicated. Standardization of protocols thereby forces industry agreement on the choice of these data points. This results in a xe2x80x9clook alikexe2x80x9d nature of competitive IEDs and tends to discourage inventive automation at the substation level.
The deregulation of the electric power industry and implementation of competitive sale of electric power has resulted in downsizing of utility personnel by the elimination of work positions and by the early retirement of experienced personnel with substitution of inexperienced personnel. In many utilities there simply are not the personnel available, experienced of otherwise, to process the vast amount of data provided them by SCADA systems.
Centralization of distribution substation operation brings about the following problems:
1) The labor intensive task of keeping computerized central control systems updated as to changes, such as in distribution line configurations and customer connections to the lines.
2) The labor intensive need for human estimation of load changes due to factors such as rapidly changing weather conditions and the effect of week ends, holidays and industry strikes on peoples working hours and their uses of electricity.
3) The continuing high costs of IED design changes, by manufacturers of IEDs, caused by the lack of agreement and standardization among the owners of substations as to the protocols used for SCADA communications.
4) The stifling of competition brought about by standardization of protocols.
5) The inability of personnel to handle the work load of centralized substation operation.
Load Tapchanging (LTC) transformers are used at power system substations to feed electrical loads (loads). Multiple transformers are sometimes used and these multiple transformers are sometimes used in combinations of parallel and independent operation. In parallel operation outgoing power lines are tied together with circuit breakers (tie breakers) to place loads on transformers operating in parallel. In independent operation, each such transformer carries loads independently of other transformers. In general all possible combinations of parallel and independent operation are found. The combinations vary from installation to installation and at any one installation may vary with time and load conditions.
Prior art paralleling schemes generally fall into two classes of schemes. The first is the master-follower scheme. In this scheme, one transformer acts as the master in responding first for a need to raise or lower one LTC tap position in controlling output voltages. When the need for a tapchange in the same direction occurs, other transformers follow the master and make tapchanges.
One second scheme uses communications by wire and equipment between transformers whereby the circulating VArs, caused by paralleled transformers when not on the same tap position, is minimized by use of paralleling devices. Another second scheme is described in U.S. Pat. No. 5,530,338, referenced hereinbelow by Robert W. Beckwith, one of the inventors herein. In this scheme the number of interconnecting wires is reduced to one wire per pair of paralleled transformers in daisy-chain fashion around a ring with no additional equipment required.
Tap switch position is available by using equipment added to LTC transformers. Alternative LTC control IEDs may keep track of tap positions using methods of U.S. Pat. No. 5,646,512 (see Reference 13 hereinbelow) by Robert W. Beckwith, one of the inventors herein.
Equipment capable of xe2x80x9chands offxe2x80x9d automation has been developed by the Beckwith Electric Co. Inc. and is termed the xe2x80x9cAutodaptive(trademark) volt/Var management systemxe2x80x9d. This utilizes inventions from U.S. Pat. No. 5,646,512, which the products follow rather closely.
The Beckwith Electric M-2667 control for LTC transformers regulates the voltage out of distribution substations as a part of the Autodaptive(trademark) system. This control is designed to provide lowest cost and highest performance. The control is capable of communicating 20 times per second in packets requiring 8 ms or less in time during otherwise unused half cycles of the power frequency.
The M-2667 control provides a vast amount of data as to its performance which is valuable in analyzing the need for maintenance. This is far beyond the amount of data space provided by standardized protocols, therefore the incentive to circumvent the protocol problem by use of the inventive wireless communications system of reference U.S. Pat. No. 5,943,202.
FIG. 7a illustrates a circuit for a prior art optically coupled RS232 port used to protect IED communications from high voltages that may exist between points within a substation during ground faults within the station. Various standards call for RS232 port withstand voltages of 1500, 2000 and 2500 Vac.
The Beckwith Electric Co. M-2600 line of controls, (IEDs), also include tapchanger controls for single phase regulators. Regulators are specialized autotransformers that change voltages +/xe2x88x9210% by means of 16 raise, 16 lower and a neutral tap position and are generally used on power distribution lines. Regulators have a voltage control bandwidth that are typically set at three volts. The most common application of regulators is in sets of three to control voltages on three phase lines. In normal operation, at times one regulator of the set may be at the low end of the band and another at the high end sometimes resulting in a three volt unbalance in phase voltages. While this much unbalance may not be damaging to three phase motors supplied by the distribution lines, undesirable power losses may result. Phase unbalance currents resulting from voltage unbalance can be analytically divided into positive, negative and zero sequence currents. Only the positive sequence currents contribute to the operation of three phase motors. The negative and zero sequence currents result in increased power losses in distribution lines, power transformers and in the motors themselves. Whenever a large number of motors is involved, costs of these power losses may be significant.
FIG. 4 of reference U.S. Pat. No. 5,943,202 shows three regulators sharing the use of a single radio for communications. This assumed that it was less costly to use two coaxial cables to save the cost of two additional radios. While this was likely correct at the time of writing reference U.S. Pat. No. 5,943,202 it may no longer be true.
The following references are used in this invention.
1: Intersil Application Note AN9804.1 dated June 1998.
2: Intersil Application Note AN9829 dated February 1999.
3: Intercil PRISM IIxe2x80x9411 MBPS descriptive bulletin.
4: xe2x80x9cAdvance Product information on the SXT810 Spread Spectrum Digital Cordless Telephone Transceiverxe2x80x9d from Level One Communications, Inc. a company being purchased by Intel.
5: xe2x80x9cA Single-Chip CMOS Direct-Conversion Transceiver for 900-MHz Spread-Spectrum Digital Cordless Phonesxe2x80x9d, a paper from Level One Communications, written by the designers of the SXT810 chip.
6: GINA 5000N/5000NV transceiver test device for 902 to 928 MHz.
7: GINA 7000N/7000NV transceiver test device for 2.404-2.478 GHz.
8: M-2667 sales brochure.
9: M-2601 (M-2665), M-2602 (M-2671), M-2603 (M-2693) sales brochures.
10: A wireless communications card for use in PCMCIA slots.
11: U. S. Pat. No. 5,943,202, TWO WAY PACKET RADIO INCLUDING SMART DATA BUFFER AND PACKET RATE CONVERSION.
12: Provisional patent application Ser. No. 60/116,984.
13: U. S. Pat. No. 5,646,512, MULTIFUNCTION ADAPTIVE CONTROLS FOR TAPSWITCHES AND CAPACITORS.
14: U. S. Pat. No. 5,530,338, LOAD TAPCHANGER TRANSFORMER PARALLELING BY DAISY CHAIN COMPARISON OF LOAD CURRENTS.
Devices using two-way packet wireless communications in accordance with U. S. Pat. No. 5,943,202, for communications in and out of electric utility substations also use wireless communications as the human interfaces for IEDs by means of hand held computers. This computer then provides the operational keys and display for use with a number of IEDs in a location. Selective wireless communications access of IEDs from wired telephone lines, coaxial cable lines, fiber optic cable lines, wireless land lines, satellite and direct Internet access telephones are included. Wireless IED to IED communications is also provided for direct peer-to-peer exchange of control and protection information within a substation without dependence on single shared apparatus. Selectively peer-to-peer groups are combined through access points accessible from land lines. When applied to sets of three power regulators, peer-to-peer communications can be utilized to reduce three phase voltage unbalance. When applied to paralleled transformers, tap position information is exchanged for paralleling. When applied to controls and protective relays, first line protection is achieved. Back up protection is then achieved by means of communications through access points using equipment common to IEDs.